1. Field of the Invention
The invention relates to the field of extruders for polymers and in particular to an improved extruder screw for extruder apparatus.
2. Description of the Related Art
The popularity of linear low density polyethylene (LLDPE) film is primarily based on its relative low cost and good mechanical properties such as tear strength, puncture resistance, tensile strength and environmental stress-crack resistance. The growth of LLDPE resin comes mostly from replacement of the polyolefin resins, low density polyethylene (LDPE) in particular.
The conversion of LDPE extruder lines to LLDPE extruder lines is not an easy proposition. LDPE screws, when used for LLDPE extrusion, require more torque and power than is available from most LDPE blown film extruder drives. This requires the LDPE extruder drive be regeared or supplied with a more powerful drive. The regeared or modified drive extruders tend to deliver melt temperatures much higher than desired for film blowing. This is due to the difference in the shear rheology of LLDPE as opposed to LDPE. LLDPE is more viscous in the range of extruder shear rates than is LDPE. Therefore it requires more energy to extrude. This energy manifests itself as increased heat causing higher temperatures.
The purpose of this invention was to develop a screw that would melt all thermoplastic materials at a lower and more controlled melt temperature, more uniform pressure and consume less power doing so.
The extruders currently used for LDPE have an elongated opening at one end of a barrel which may be heated or cooled at specified locations along its length. An extruder screw is positioned within the barrel and runs the length of the barrel. The screw has a helical thread or flight on its surface which is in close fit with the cylindrical surface of the barrel. The screw is rotated about its axis to work the plastic material and force it toward the outlet end of the extruder.
An extruder screw usually has a plurality of sections such as a metering section, a feed section and a transition section. The feed section is usually provided beneath the opening in the hopper which feeds the polymer to be extruded into the extrusion apparatus. The feed section will normally extend into the barrel of the extruder. The screw diameter, or "root", in the feed section is usually the smallest on the screw. This allows the feed section to pull large amounts of polymer powder or pellets into the extruder so they may be compressed in the forward screw portions. This overfill is desirable as the extruder obtains optimum performance when the space between two adjacent flights is completely filled with material with a minimal number of voids.
In a common extruder screw there is a transition section after the feed section. In the transition section the root increases thus decreasing the volume of space which is available for the polymer and thereby compressing it.
The transition section is followed by a metering section of constant root. A large part of the shearing and polymer melt occurs in the section as the polymer moves along the screw length.
The shearing of the material as it moves along the extruder screw through the barrel generates heat and melts the polymer. Most of the melting occurs at the barrel surface. As the polymer melts and the flight behind the polymer moves forward, the melted polymer flows down the face of the flight, forward along the root of the screw and up the back of the lead flight. This will sometimes cause a pocket of polymer solid to become encapsulated within the melted polymer pool which is flowing around the perimeter formed by the two adjacent flights, the screw surface and the barrel surface.
The conventional feed-transition-metering type of extruder screw was designed for the general extrusion of all thermoplastics. However, when the same type of extruder screw is used with LLDPE the power consumption and exit temperatures increase while maximum output or throughput decreases. In order to improve the performance of the extruder without regearing, a screw was developed having two transition sections and two metering sections. U.S. Pat. No. 4,357,291 to Miller et al. discloses such a screw. Such a screw provides a separate compression and metering section; however, there is no assurance of proper and complete melt as the solids bed may become encapsulated.
There has been proposed an extruder screw with a barrier flighted section. The barrier flight assists the extrusion process by separating the melt pool from the solids bed. This leaves the solids bed to be worked and heated to melt it.